The research proposed will continue and complete ongoing investigations on the trophic influence of nerve on muscle. It is hoped to elucidate the possible existence of a trophic substance. Newt muscle will be organ cultured in a defined medium to which various additions can be made. The effects of additions such as brain homogenate, neural tissue, and c-AMP on the acetylcholine sensitivity of the muscles will be investigated. Cholinergic sensitivity will be measured by the binding of radioactive alpha-bungarotoxin. A technique for the electron-microscopic localization of cholinergic receptors will also be developed using alpha-bungarotoxin covalently linked with horse-radish peroxidase. The peroxidase reaction products can be detected by histochemical methods.